1. Field of the Invention
The present invention relates to a method of forming a titanium (Ti) film to be used as a contact metal film or an adhesion layer for a semiconductor device by a chemical vapor deposition (CVD) process.
2. Description of the Related Art
Recently, most semiconductor integrated circuits are fabricated in a circuit configuration of a multilevel structure to cope with market demand for high-density and large-scale integration. Accordingly, techniques for filling contact holes for the electrical connection of semiconductor devices formed in a lower layer to wiring lines formed in upper wiring layers, and via holes for the electrical connection of wiring lines in different wiring layers have become important.
Generally, aluminum (Al), tungsten (W), aluminum-base alloys and tungsten-base alloys are used for filing contact holes and via holes. If a contact hole (or a via hole) is filled with such a metal or an alloy, which is in direct contact with an Al wiring line or a silicon (Si) substrate in a lower layer, it is possible that an alloy of silicon and aluminum is produced in the boundary owing to the diffusion of Al and Si. Such an alloy has a high resistivity and is undesirable in view of need to reduce the power consumption and to increase the operating speed of integrated circuits If W or a W-base alloy is used for filling a contact hole, WF.sub.6 gas as a source gas for depositing W or the W-base alloy tends to deteriorate the electrical properties of the Si substrate.
To avoid such problems, a barrier layer is formed on surfaces defining contact holes or via holes before filling the contact holes and the via holes with the filing metal. Generally, a two-layer barrier layer consisting of a Ti film and a TiN (titanium nitride) film is used as the barrier layer. It has been the usual way to form such a barrier layer by a physical vapor deposition (PVD) process. However, a PVD film has a poor coverage and is incapable of meeting the requirements of tight design rule, the reduction of the width of lines and diameter of openings and the increase of aspect ratio to meet the recent demand for the enhancement of the level of integration and the miniaturization of IC chips.
Recently, Ti films and TiN films have been formed by a CVD process capable of forming such films in a better film quality than the PVD process. The Ti film serving as a contact metal film is formed by a plasma CVD process. Usually, the plasma CVD process for forming a Ti film uses TiCl.sub.4 gas as a source gas, H.sub.2 gas and Ar gas. During the plasma CVD process, Si forming the substrate and Ti contained in the source gas interact and Si diffuses in the Ti layer, deteriorating the morphology of a TiSi.sub.X (typically, TiSi.sub.2) interfacial layer formed between the Si substrate and the Ti layer. The diffusion of Si into the Ti layer is liable to cause junction leakage when the Ti layer is used for filling a contact hole and affects adversely to electrical connection when the same is used for filling a via hole.
The aspect ratio of contact holes and via holes formed in SiO.sub.2 insulating films has increased with the progressive miniaturization of IC chips. Therefore, the Ti film is required to be formed in a satisfactory step coverage.
The present invention has been made in view of the above and it is therefore an object of the present invention to provide a method of forming a Ti film by CVD, capable of forming a Ti film in minute holes formed in an insulating film in a satisfactory step coverage without deteriorating the morphology of a TiSi.sub.X interfacial layer formed between the Ti film and a Si base.